1. Field of the Invention
This invention relates to an apparatus for detecting a misfire in an internal combustion engine by detecting an ionic current which flows in a spark plug disposed in a combustion chamber of the internal combustion engine.
2. Description of the Related Art
In an internal combustion engine, a mixture of fuel and air is compressed and the mixture is ignited by an electric spark generated due to application of high voltage to a spark plug disposed in the combustion chamber. A state where the mixture is not ignited is called "misfire". In the foregoing case, a satisfactory output from the internal combustion engine cannot be obtained. In addition, the mixture containing fuel in a large quantity is introduced into the exhaust system, thus raising a problem in that the muffler and the like are corroded by the mixture. Therefore, misfires must be detected in order to issue an alarm to a driver.
As an apparatus for detecting a misfire in an internal combustion engine, a circuit has been available which detects a misfire by detecting an ionic current which flows in a spark plug disposed in the combustion chamber. When combustion takes place in the combustion chamber, molecules in the combustion chamber are ionized. When voltage is, through the spark plug, applied into the combustion chamber which is in the ionized state, a small electric current flows, which is called an "ionic current". Since the ionic current is greatly diminished if a misfire takes place, occurrence of the misfire can be discriminated by detecting this change in ionic current.
FIG. 7 illustrates a conventional apparatus of the foregoing type for detecting a misfire in an internal combustion engine which has been disclosed in, for example Japanese Patent Laid-Open No. 4-191465.
Referring to FIG. 7, reference numeral 1 represents an ignition coil, 1a and 1b respectively represent a primary coil and a secondary coil of the ignition coil 1, 2 represents a spark plug disposed in a combustion chamber 20, the spark plug 2 being connected to the negative terminal of the secondary coil 1b. The primary coil 1a has a positive terminal connected to a power source 4 and a negative terminal connected to the collector of a transistor 5 which switches the electric current. The emitter of the transistor 5 is connected to ground and the base of the transistors is controlled by a control unit (not shown) which controls combustion.
Reference numeral 8 represents a misfire detection circuit, 9 represents a capacitor connected to the positive terminal of the secondary coil 1b, 10 represents a Zener diode connected between the positive terminal of the secondary coil 1b and ground to set the voltage for charging into the capacitor 9, and 11 represents a diode connected such that its portion adjacent to the capacitor 9 is the anode thereof, the diode 11 being connected between the low potential side of the capacitor 9 and ground. Reference numeral 12 represents a resistor.
In the circuit having the foregoing structure, the control unit (not shown), at the ignition timing for the internal combustion engine, performs control so that the transistor 5, which has been turned on, is rapidly turned off. At this time, the primary current flowing in the ignition coil 1 is rapidly decreased and, thus, the counter electromotive force of the coil generates high voltage. The voltage generated on the primary side is amplified in accordance with the coil ratio between the primary coil 1a and the secondary coil 1b, the amplified voltage appearing on the secondary side of the ignition coil 1. As a result, the spark plug 2 is applied with voltage of, for example, about -10 KV to about -25 KV.
In the circuit shown in FIG. 7, energy at the ignition timing is used to accumulate charges in the capacitor 9, the charges being sufficient to detect the ionic current. The voltage supplied from the capacitor 9 is used to detect the ionic current immediately after ignition. The electric current, at the ignition timing, flows in a direction indicated by an arrow 2c shown in FIG. 7, thus causing the spark plug 2 to discharge electricity. Thus, the mixture in the combustion chamber 20 is ignited. The discharge current charges the capacitor 9, and therefore, the capacitor 9 is charged to a voltage level limited by the Zener diode 10.
When the igniting electric current flowing in the direction indicated by the arrow 2c is decreased to zero, the voltage maintained in the capacitor 9 is applied to the spark plug 2. At this time, if combustion takes place normally in the combustion chamber 20, an ionic current flows through the resistor 12 in a direction indicated by an arrow 2d. Therefore, the resistor 12 causes the voltage to be lowered and lowering of the voltage is, as a detection signal, used to discriminate whether or not a misfire has taken place. If a misfire takes place, the flowing ionic current is greatly diminished and therefore substantially no voltage caused from this appears as the output.
The apparatus for detecting a misfire in an internal combustion engine has a problem in that the misfire detection involves an error due to stray capacitance and the like, as will be described subsequently.
That is, the misfire detection circuit is, together with an ignition coil and the like, disposed in the engine compartment of an automobile in a variety of methods depending upon the structure of the engine or the like. For example, the length from the ignition coil 1 to the spark plug 2 shown in FIG. 7 is sometimes about 2 m in a case where it is long. If the length of wiring is long, stray capacitance is generated between the foregoing wiring and another wiring, in particular, the ground, that has another potential.
Assuming that the stray capacitance with respect to the ground is Cf [F](farad) in a case of the circuit shown in FIG. 7, a series circuit consisting of the stray capacitance Cf, the capacitor 9 and the resistor 12 is formed. The operation of the series circuit is affected considerably by a charging/discharging time constant determined by the resistance value of each of the stray capacitance Cf and the resistor 12. In particular, a problem rises in that the time width of the noise signal is enlarged. Specifically, decaying of noise currents of 100 .mu.sec (microsecond) and 10 mA (milliampere) to 1 .mu.A (microampere) or smaller that is free from a problem as compared to the ionic current requires a time period of about 1 msec (millisecond) if the stray capacitance Cf is 500 pF (picofarad) and the resistor 12 is 200 K.OMEGA. (kilohm), thus causing the noise current waveform to be widened to about 10 times. Therefore, there rises a possibility that noise is erroneously detected as the ionic current.
To overcome the foregoing problem, it might be considered feasible to reduce the resistance value of the resistor 12 or to decrease the stray capacitance. If the resistance value is reduced, the sensitivity to detect a misfire is deteriorated, thus raising a problem in that detection cannot be performed in a low rotational region in which the ionic current is decreased. The decrease of the stray capacitance considerably limits the place in which the detection circuit is disposed and the method of the disposition.
In view of the foregoing, a circuit for detecting a misfire in an internal combustion engine has been suggested which is capable of preventing erroneous detection taking place due to the stray capacitance and the reliability of which can be improved (refer to Japanese Patent Application No. 6-8880 filed on Jan. 28, 1994).
FIG. 8 is a structural view showing a circuit equivalent to an apparatus for detecting a misfire in an internal combustion engine of the foregoing type that is capable of preventing erroneous detection.
Referring to FIG. 8, the same elements as those shown in FIG. 7 are given the same reference numerals and their description is omitted here.
Novel reference numerals will now be described. Reference numerals 2a and 2b represent spark plugs of a simultaneous-ignition type which produce electric sparks by using high voltage generated at the two electrodes of the secondary coil 1b of the ignition coil 1. Reference numeral 3 represents a voltage-resistible diode, the cathode of which is connected to the spark plug 2b, the anode of which is connected to the positive terminal of the capacitor 9 in the misfire detection circuit 8 and which detects an ionic current. The collector of the transistor 5 for switching the electric current is connected to the negative terminal of the primary coil 1a of the ignition coil 1 and as well as the capacitor 9 of the misfire detection circuit 8 is connected to the same through the resistor 6 and the high-voltage diode 7. Thus, positive bias voltage is applied to the capacitor 9 so that a charging current is supplied from the primary coil 1a of the ignition coil 1 through the resistor 6 and the high-voltage diode 7.
New reference numeral 13 represents a second diode as contrasted with the first diode which is the diode 11 having the anode connected to the low potential side of the capacitor 9 and the cathode connected to ground. The second diode 13 has a cathode connected to the low potential side of the capacitor 9 and an anode connected to the earth. Reference numeral 14 represents an operational amplifier (hereinafter called an "op-amplifier) having an inverting input connected to the anode of the diode 11 and a non-inverting input connected to ground, the operational amplifier 14 having a feedback resistor 15 connected between the inverting input and the output.
In a circuit structured as shown in FIG. 8, a control unit (not shown), at the ignition timing for the internal combustion engine, performs control so that the transistor 5, which has been turned on, is rapidly turned off. At this time, the primary current flowing in the ignition coil 1 is rapidly decreased and, thus, the counter electromotive force of the coil generates high voltage. The voltage generated on the primary side is amplified in accordance with the coil ratio between the primary coil 1a and the secondary coil 1b, the amplified voltage appearing on the secondary side of the ignition coil 1. As a result, the spark plug 2a is applied with negative voltage of, for example, about -10 KV to about -25 KV, while the ignition coil 2b is applied with positive voltage of, about 10 KV to 25 KV.
In the circuit shown in FIG. 8, the electric current flowing from the primary side of the ignition coil 1 to the capacitor 9 through the resistor 6 and the high-voltage diode 7 charges the capacitor 9 in a period in which high voltage is generated from the primary side of the ignition coil 1 due to the counter electromotive force, the capacitor 9 being charged to a voltage level (for example, the Zener voltage of the Zener diode 10: VZ=50 V) which is limited by the Zener diode 10. Thus, charges sufficient to detect the ionic current are accumulated in the capacitor 9. In accordance with the voltage charged into the capacitor 9, the ionic current flowing through the secondary side of the ignition coil 1 is detected.
FIG. 9 shows the waveforms of portions S1 and S2 of the circuit shown in FIG. 8. The waveform S1 represents the base potential of the transistor 5 for controlling the electric current flowing in the primary coil 1a of the ignition coil 1 and S2 represents the negative terminal potential of the primary coil 1a.
The transistor 5 is turned on in an ON period in which the electric current is caused to flow in the primary coil 1a and turned off in an OFF period in which the electric current in the primary coil 1a is stopped. When the transistor 5, which has been turned on, is turned off, the counter electromotive force of the coil raises the voltage of S2, which is the negative terminal of the primary coil 1a, to VH=about 300 V. The raised voltage is the same as the resistible voltage between the collector and the emitter of the transistor 5. In a period in which the counter electromotive force is generated, an electric current flows in the capacitor 9 through the resistor 6 and the diode 7. Thus, the capacitor 9 is charged to about Zener voltage VZ (for example, 50 V) which is limited by the Zener diode 10; and the voltage V2 at the negative terminal S2 of the primary coil 1a of the ignition coil 1 is lowered to about the value of the Zener voltage VZ, strictly the voltage V2 being lowered to a value which is the result of addition of the voltage drop taking place due to the resistor 6 and the forward-directional voltage of the diode 7.
The high voltage VH generated at the primary coil 1a of the ignition coil 1 is amplified in accordance with the coil ratio between the primary coil 1a and the secondary coil 1b of the ignition coil 1, the high voltage VH being applied to the spark plug 2a connected to the negative terminal of the secondary coil 1b so that the spark plug 2a is ignited. The electric current, at the ignition timing, flows in a direction indicated by an arrow 2c so that a spark is generated by the spark plug 2a and discharge takes place. Thus, the mixture in the combustion chamber 20 is ignited. After the capacitor 9 has been charged completely, a state is realized in which the voltage accumulated in the capacitor 9 is applied to the spark plug 2a. If combustion is being performed in the combustion chamber 20 at this time, then an ionic current flows on the secondary side of the ignition coil 1 in a direction indicated by an arrow 2d.
The ionic current is, by the misfire detection circuit 8, converted into voltage. In accordance with whether or not the voltage obtained by the conversion exceeds a threshold, whether or not a misfire has taken place is discriminated. That is, if a misfire takes place, namely, if no combustion is performed, a very small electric current flows and, therefore, substantially no voltage due to this appears in the output. Note that the voltage at an end S2 of the primary coil 1a of the ignition coil 1 is gradually decreased after the capacitor 9 has discharged electric power to reach the battery voltage VBAT of the power source 4. When the transistor 5 is then controlled to be turned off, the voltage is made to be zero.
The voltage on the low potential side of the capacitor 9 is the voltage of the inverting input of the inverse amplifier composed of the foregoing operational amplifier 14 and the resistor 15. In a case where the operational amplifier 14 is operated normally, the inverting input voltage and the non-inverting input voltage are the same, thus, resulting in 0 V. The case where the operational amplifier 14 is not performed normally is a case where the electric current flows in the direction indicated by the arrow 2c and a case where the electric current flowing in the direction indicated by the arrow 2d is too large and therefore the output from the operational amplifier 14 is saturated. In the case where the electric current flows in the direction indicated by the arrow 2c, that is, in a case where the capacitor 9 is in a charged state, the charging current flows from the primary coil 1a to the capacitor 9 through the resistor 6 and the diode 7, so that the voltage on the low potential side of the capacitor 9 is the forward-directional voltage (0.7 V) of the first diode 11. In the case where the electric current flowing in the direction indicated by the arrow 2d is too large and therefore the output from the operational amplifier 14 is saturated, the second diode 13 is made conductive. Thus, the voltage at the low potential side of the capacitor 9 is lowered by a degree corresponding to the forward-directional voltage. In a case where the operational amplifier 14 is operated normally, the ionic current appears as the voltage drop of the resistor 15 and is converted into a signal based on ground, the signal being transmitted.
By employing the foregoing circuit structure, the low-potential side of the capacitor 9 involves a small voltage change with respect to change in the electric current. If the operational amplifier 14 is normal, the apparent voltage on the low-potential side of the capacitor 9 is constantly 0 V. If the operation of the operational amplifier 14 is not normal, the same is made to be constant which is the forward-directional voltage of the diode. That is, the impedance of the detection circuit viewed from the low-potential side of the capacitor 9 is extremely low. The foregoing operation reduces the impedance of the circuit without deterioration in the current/voltage conversion characteristic (the detectable sensitivity) of the ionic current. As a result, the durable amount against the erroneous operation occurring due to the stray capacitance and the impedance of the circuit can be improved significantly.
Specifically, as compared with the conventional degree of occurrence of erroneous operations which have taken place if the stray capacitance is about 200 pF (picofarad), the operation can be performed if the capacitance is about 2000 pF while maintaining a similar detectable sensitivity. Thus, a satisfactory large operational margin can be obtained with respect to the stray capacitance that takes place practically.
However, the apparatus for detecting a misfire in an internal combustion engine shown in FIG. 8 has the following problem.
That is, since the capacitor 9 for detecting the ionic current is charged with electric currents supplied from the primary coil 1a of the ignition coil 1, the capacitor 9 is electrically charged regardless of the ignition of the secondary side. Therefore, when the spark plug 2a discharges electricity, the ionic current can be detected. Thus, an erroneous detection is sometimes performed due to the change in the discharging voltage on the secondary side.